New moon
When he and Remus had been students at Hogwarts, they'd usually started plotting the next Marauder adventure two weeks ahead. Now, Sirius had much greater plans in the making. For the first time in over twelve years, he got to sleep in his own bed (and also, without any blood-purist Slytherin family members around) yet he was up all night. Thoughts were running through his mind. It was also a lot warmer than what he got used to. Eventually he went downstairs, into the cold.
His gaze fell on the huge dark blurr in front of the cold fireplace. Daire's familiar rattling breath and chilly cold filled the living-room. Now that he was asleep, the dementor's outline had faded. Presumably, Sirius wondered, this was how he'd be perceived by a squib when awake.
For just an eyeblink's time, the pile of black cloak solidified, and the loud breathing was replaced with a cautious wind when Sirius came too close. Then Daire fell back asleep, having identified the wizard as his benefactor. The one who had saved his life with that diadem. The one who had invited him to stay at 12, Grimmauld Place, when it had become clear that he wouldn't survive in Azkaban. Sirius recalled Vaqqu explaining to him that the prison was too crowded with dementors for the wounded to feed. The wizard had immediately offered his family's empty house, and had bitterly commented that with Daire staying there, he'd maybe have someone to greet him when he'd get home.
He hadn't hoped it'd happen this soon. Yet he'd been moved under house arrest two days after half Hogsmeade had witnessed a hippogriff chasing a doubtlessly-alive Peter Pettigrew through the village. Sirius had honestly appreciated Moony for that achievement. But how could they trap the little traitor for good?
And this was only one of the lesser problems. After they had told Dumbledore what had transpired in the Room of Requirement, the director of Hogwarts had confirmed the existence of objects containing a wizard's soul. Furthermore, he had told them a secret: a similar item had almost caused several deaths at the school the previous year. And that object, a diary, had once belonged to the man only Dumbledore would refer to by his name.
Sirius's gaze fell on the suspiciously empty locket Daire had found in the kitchen before the house owner had been allowed home. If there were objects like that in his own house, what should he expect to find in the wider family's vaults at Gringotts?
He opened a dirty window, and let the fresh September air in. He looked up at the starry sky. It was a new moon, time for crazy new plans. It was his first full night outside Azkaban, and nobody was screaming in the cells next to him.
Realization hit him as he remembered accusations he'd heard so often. Wormtail might give himself up if Padfoot made the Unbreakable Vow to defend him in court.
Crazy plan - check.